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Chapter 19

Reproductive Development

 

The onset of flowering in plants represents a major developmental shift in the plant life cycle. This shift is controlled by environmental cues, physiological changes, and alterations in gene expression. Many of the genes involved in the induction of flowering have been identified in model organisms. At the shoot apex, a small number of floral meristem identity genes control the positioning and identity of flowers. In Arabidopsis, flowers are organized into four whorls. The organs on each of these whorls are specified by combinations of transcription factors, which are encoded by MADS box genes. These genes originally were identified by studying homeotic mutant phenotypes in which whorl identity is altered. The MADS box genes that control the patterning of floral organ identity evidently are conserved among angiosperms.
      Gametes must be formed to complete the plant life cycle. In the anthers, sporogenous cells undergo meiosis and each resulting haploid cell eventually develops into a male gametophyte (pollen grain). In the ovules, sporogenous cells undergo meiosis and eventually form the female gametophyte (embryo sac). Many genes are expressed only during gametophyte development. When pollen grains contact the stigmatic tissues of the gynoecium, they hydrate and then extrude a pollen tube, growing though stylar tissue to deliver the two sperm cells to the embryo sac. Typically, one sperm cell nucleus fuses with the egg cell nucleus and the other fuses with the nuclei of the central cell. Genetic analysis of mutants and molecular analysis of gene expression have contributed to our understanding of gametophyte development and pollen–pistil interactions.
      Seed development proceeds from the single-celled fertilized egg to a multicellular structure comprising an embryo, which is the next generation of the plant; the seed coat (testa); and a nutritive endosperm that persists in the mature dry seed of some species. Many genes are involved in the regulation of seed development. The roles of some of these genes have been elucidated by using developmental mutants. An important aspect of seed development is the deposition of the stored reserves—carbohydrates, proteins, and oils—that are utilized after germination to support early seedling growth. The terminal phase of seed development is desiccation, which results in a mature dry seed. On subsequent imbibition of water, metabolism is renewed as the seed commences germination, which is completed with the emergence of the radicle.


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